1. Field of the Invention
This invention relates generally to display apparatus for producing optical images, and more particularly to a method and apparatus using an array of sets of grating light valves and a plurality of colored light sources to provide a multicolor image that can be directly viewed or projected onto a screen.
2. Brief Description of the Prior Art
Devices which modulate a light beam, e.g. by altering the amplitude, frequency or phase of the light, find a number of applications. An example of such a device is a spatial light modulator (SLM) which is an electronically or optically controlled device that consists of one or two-dimensional reconfigurable patterns of pixel elements, each of which can individually modulate the amplitude, phase or polarization of an optical wavefront.
These devices have been extensively developed, particularly for applications in the areas of optical processing and computing. They can perform a variety of functions such as: analog multiplication and addition, signal conversion (electrical-to-optical, incoherent-to-coherent, amplification, etc.), nonlinear operations and short term storage. Utilizing these functions, SLMs have seen many different applications from display technology to optical signal processing. For example, SLMs have been used as optical correlators (e.g., pattern recognition devices, programmable holograms), optical matrix processors (e.g., matrix multipliers, optical cross-bar switches with broadcast capabilities, optical neural networks, radar beam forming), digital optical architectures (e.g., highly parallel optical computers) and displays.
The requirements for SLM technology depend strongly on the application in mind: for example, a display requires low bandwidth but a high dynamic range while optical computers benefit from high response times but don't require such high dynamic ranges. Generally, systems designers require SLMs with characteristics such as: high resolution, high speed (kHz frame rates), good gray scale high contrast ratio or modulation depth, optical flatness, VLSI compatible, easy handling capability and low cost. To date, no one SLM design can satisfy all the above requirements. As a result, different types of SLMs have been developed for different applications, often resulting in trade-offs.
A color video imaging system utilizing a cathode ray device with a target comprising an array of electrostatically deflectable light valves is disclosed in U.S. Pat. No. 3,896,338 to Nathanson et al. The light valve structure and the arrangement of light valves as an array permits sequential activation of the light valves in response to a specific primary color video signal. The light valves are arranged in three element groupings, and a schlieren optical means is provided having respective primary color transmissive portions through which the light reflected from the deflected light valves is passed to permit projection of a color image upon a display screen.
Texas Instruments has developed a "Deformable Mirror Device (DMD)" that utilizes an electromechanical means of deflecting an optical beam. The mechanical motions needed for the operation of the DMD result in bandwidths limited to tens of kilohertz. However, this device generally provides better contrast ratios than the technologies previously described, provides acceptable "high resolution" and is compatible with conventional semiconductor processing techniques, such as CMOS.
Nematic and ferroelectric liquid crystals have also been used as the active layer in several SLMs. Since the electro-optic effect in liquid crystals is based on the mechanical reorientation of molecular dipoles, it is generally found that liquid crystals are faster than the DMD-type devices. Modulators using ferroelectric liquid crystals have exhibited moderate switching speeds (150 .mu.sec to 100 nsec), low-power consumption, VLSI compatible switching voltages (5-10 V), high extinction ratios, high resolution and large apertures. However, these devices suffer from the drawbacks of limited liquid crystal lifetimes and operating temperature ranges. In addition, the manufacturing process is complicated by alignment problems and film thickness uniformity issues.
Magneto-optic modulation schemes have been used to achieve faster switching speeds and to provide an optical pattern memory cell. Although these devices, in addition to achieving fast switching speeds, can achieve large contrast ratios, they suffer from a low (&lt;10%) throughput efficiency and are, therefore, often unsuitable for many applications.
The need is therefore for a light modulation device which overcomes these drawbacks.
Beside SLMs, another area of use of light modulators is in association with fiber optics apparatus. Fiber optic modulators are electronically controlled devices that modulate light intensity and are designed to be compatible with optical fibers. For high speed communication applications, lithium niobate (LiNbO.sub.3) traveling wave modulators represent the state-of-the-art, but there is a need for low power, high efficiency, low loss, inexpensive fiber optic modulators, that can be integrated with silicon sensors and electronics, for data acquisition and medical applications. A typical use of a modulator combined with fiber optic technology, for example, is a data acquisition system on an airplane which consists of a central data processing unit that gathers data from remote sensors. Because of their lightweight and electro-magnetic immunity characteristics, fiber optics provide an ideal communication medium between the processor and the sensors which produce an electrical output that must be converted to an optical signal for transmission. The most efficient way to do this is to have a continuous wave laser at the processor and a modulator operating in reflection at the sensor. In this configuration, it is also possible to deliver power to the sensor over the fiber.
In this type of application the modulator should operate with high contrast and low insertion loss to maximize the signal to noise ratio and have low power consumption. It should further be compatible with silicon technology because the sensors and signal conditioning electronics used in these systems are largely implemented in silicon.
Another use of a modulator combined with fiber optic technology is in the monitoring of sensors that are surgically implanted in the human body. Here optical fibers are preferred to electrical cables because of their galvanic isolation, and any modulator used in these applications should exhibit high contrast combined with low insertion loss because of signal to noise considerations. Furthermore, as size is important in implanted devices, the modulator must be integratable with silicon sensors and electronics.
Modulators based on the electro-optic, Franz-Keldysh, Quantum-Confined-Stark or Wannier-Stark effect in III-V semiconductors have high contrast and low insertion loss, but are expensive and not compatible with silicon devices. Waveguide modulators employing glass or epi-layers on silicon, require too much area and too complex fabrication to be easily integratable with other silicon devices. Silicon modulators that do not employ waveguides and that are based on the plasma effect, require high electrical drive power and do not achieve high contrast.
A need therefore exists for improved light modulator apparatus having low power requirements, high efficiency, low loss, low cost and compatibility with silicon technology.
A need also exists for a multicolor display device using light modulator technology of the type described herein.